1. Field of the Invention
The present invention relates to an electroluminescent display device, and in particular, to transistors constructing the circuit structure in the pixel section of an electroluminescent display device.
2. Description of the Related Art
An electroluminescence (hereinafter referred to as EL) display device which uses an EL element which is a self-illuminating element as an illumination element in each pixel has attracted a strong interest as an alternative display device for a display device such as a liquid crystal display device (LCD) and a CRT because the EL display device has advantages such as thin width and low power consumption, in addition to the advantage of being self-illuminating. Such an EL display device has thus been researched.
In particular, there is a high expectation for an active matrix type EL display device in which a switching element such as, for example, a thin film transistor for individually controlling an EL element is provided for each pixel and EL elements are controlled for each pixel, as a high resolution display device.
FIG. 1 shows a circuit structure for one pixel in an active matrix type EL display device having m rows and n columns. In the EL display device, a plurality of gate lines GL extend on a substrate in the row direction and a plurality of data lines DL and power supply lines VL extend on the substrate in the column direction. Each pixel has an organic EL element 50, a switching TFT (first TFT) 10, an EL element driving TFT (second TFT) 20, and a storage capacitor Cs.
The first TFT 10 is connected to the gate line GL and data line DL, and is turned on by receiving a gate signal (selection signal) on its gate electrode. A data signal which is being supplied on the data line DL at this point is then held in the storage capacitor Cs connected between the first TFT 10 and the second TFT 20. A voltage corresponding to the data signal is supplied to the gate electrode of the second TFT 20 via the first TFT 10. The second TFT 20 then supplies a current, corresponding to the voltage value, from the power supply line VL to the organic EL element 50. In this manner, the organic EL element in each pixel is illuminated at a brightness based on the data signal, and a desired image is displayed.
The organic EL element is a current-driven element which is illuminated by supplying a current to an organic emissive layer provided between a cathode and an anode. The data signal output onto the data line DL, on the other hand, is a voltage signal with an amplitude corresponding to the display data. Thus, conventionally, in order to accurately illuminate the organic EL element by such a data signal, in an organic EL display device, a first TFT 10 and a second TFT 20 are provided in each pixel.
The display quality and reliability of the organic EL display devices described above remain insufficient, and the characteristic variations in the first and second TFTs 10 and 20 must be dissolved. In particular, reduction in characteristic variation in the second TFT 20 for controlling the amount of current supplied from the power supply line VL to the organic EL element 50 is desired, because such variation directly causes variation in the illumination brightness.
Moreover, it is preferable to construct the first and second TFTs 10 and 20 from a polycrystalline silicon TFT which has quick operation speed and which can be driven by a low voltage. In order to obtain a polycrystalline silicon, an amorphous silicon is polycrystallized by laser annealing. Because of various reasons such as, for example, energy variation in the irradiating laser at the irradiation surface, the grain size of the polycrystalline silicon is not uniform. When grain size is not uniform, in particular around the TFT channel, there is a problem in that the on-current characteristic or the like of the TFT may also vary.
The present invention is conceived to solve the above problem, and one object of the present invention is to provide an active matrix type organic EL panel capable of illuminating each illumination pixel at a uniform brightness by alleviating the characteristic variations of the TFT which controls the organic EL element.
According to one aspect of the present invention, there is provided an active matrix type display device in which each of a plurality of pixels arranged in a matrix form comprises at least an element to be driven and an element driving thin film transistor for supplying power from a driving power supply to the element to be driven; wherein each pixel region of the plurality of pixels has one of the sides in the row direction or column direction of the matrix longer than the other side; and the element driving thin film transistor is placed so that its channel length direction is along the longer side of the pixel region.
According to another aspect of the present invention, in the display device, it is preferable that in the pixel region the side along the column direction of the matrix is longer than the side along the row direction of the matrix; and that the element driving thin film transistor is placed so that its channel length direction is along the column direction.
According to another aspect of the present invention, there is provided a semiconductor device comprising at least one element driving thin film transistor for supplying driving current from a power supply line to a corresponding element to be driven; and a switching thin film transistor for controlling the element driving thin film transistor based on data supplied when selected; wherein the element driving thin film transistor is placed so that its channel length direction is along the extension direction of a data line for supplying the data signal to the switching thin film transistor.
By employing such a configuration, it is possible to increase the channel length of the element driving thin film transistor for supplying power to the element to be driven, and, to thereby improve reliability characteristics of the transistor such as, for example, its durability. In addition, the characteristic of the element driving thin film transistors each provided for an element to be driven can be averaged, and, thus, the variation in the illumination brightness among the elements can be inhibited even when the element to be driven is an emissive element which has different illumination brightness depending on the supplied power. Moreover, the configuration facilitates efficient placement of a plurality of element driving thin film transistors, for example, with sufficient channel length with respect to one element to be driven, in parallel or in series within a pixel and, thus, it is possible to increase the illumination region in a case where the element to be driven is an emissive element.
According to another aspect of the present invention, in the semiconductor device or display device, it is preferable that the element driving thin film transistor is formed so that its channel length direction is along the scan direction of a line pulse laser for annealing the channel region of the transistor.
In this manner, by coinciding the channel length direction of the element driving thin film transistor and the scan direction of the laser annealing, the difference in the transistor characteristics from the element driving thin film transistors for supplying power to other elements to be driven can be reliably reduced.
In laser annealing, the laser output energy tends to vary. The variation includes a variation in the pulse laser within an irradiation region and variation among the shots. In many cases, the element driving thin film transistor which is used for a semiconductor device such as, for example, an active matrix type display device, is designed so that the channel length is significantly greater than the channel width. By placing the element driving thin film transistor along the longer side of the pixel region or forming the element driving thin film transistor along the column direction or the extension direction of the data line, the channel length of the element driving thin film transistor can easily be increased to a sufficient length. By setting the scan direction of the laser to almost coincide with the channel length direction of the element driving thin film transistor, that is, by setting the scan direction of the laser so that the longitudinal direction of the laser irradiation region crosses the channel in the width direction, the device can easily be adjusted so that the entire channel region of one element driving thin film transistor is not simultaneously annealed by a single shot. This can be easily achieved by, for example, setting the channel length of the element driving thin film transistor to be longer than one moving pitch of a pulse laser. Thus, in a case where a plurality of elements to be driven is formed on the same substrate and a plurality of element driving thin film transistors for supplying power to the plurality of elements is formed, it is possible to laser anneal the active layers of the thin film transistors by a plurality of shots, resulting in the transistors to be subjected more uniformly to the energy variation among the shots and reliable averaging of the characteristics among the thin film transistors. In this manner, for example, in an organic EL display device which uses an organic EL element as the element to be driven, which uses an organic compound as the emissive layer, variation in the illumination brightness among the organic EL elements provided for different pixels can be significantly reduced.
According to another aspect of the present invention, in the semiconductor device, it is preferable that the channel length direction of the element driving thin film transistor does not coincide with the channel length direction of the switching thin film transistor.
The switching thin film transistor is placed near the section where the selection line for selecting the transistor and the data line for supplying a data signal cross each other. In many cases, the switching thin film transistor is placed so that its channel length direction is approximately parallel to the extension direction of the selection line. In such a case, by placing the element driving thin film transistor so that its channel length direction is different from that of the switching thin film transistor, the channel length of the element driving thin film transistor can easily be increased.
According to another aspect of the present invention, it is preferable that the element to be driven is an organic electroluminescence element which employs an organic compound as an emissive layer. Although such an organic EL element has high brightness and wider selection ranges for the illumination color and material, because the organic EL element is current driven, variation in the amount of supplied current causes a variation in the illumination brightness. By using the circuit structure of the pixel or placement as described above, it is possible to easily maintain uniformity of the supplied current. In addition, by employing the placement and structure of the contact points as described above, the aperture ratio can be increased and the element layer such as the emissive layer can be formed on a flat surface, and a more reliable element can be obtained.